Sintered shaped body, whose surface comprises a porous layer and a method for the production thereof

ABSTRACT

Porous coatings on high-performance ceramics attempt to combine the mechanical and thermal characteristics, which fulfil stringent demands, of the substrate material with the advantageous properties of coating materials. The subsequent application of layers of this type to the pre-sintered substrate produces unsatisfactory results in several areas of use with regard to possible layer thickness, porosity and adhesion. According to the invention, a shaped body consisting of a sintered, inorganic material, whose surface comprises a porous layer is produced in such a way that the base body is first formed as a green body. A layer in the form of a suspension, also containing an inorganic material, is then applied to the surface or to one section of the surface of the base body. A predetermined fraction of a pore-forming substance is mixed with at least the material of said layer and the green body with its applied layer is subjected to the thermal treatments required for producing a monolithic sintered body.

This is a 371 application of PCT/EP01/02841 filed Mar. 14, 2001 whichclaims priority to German Patent Application 100 15 614.2 filed Mar. 29,2000.

The invention relates to a shaped body according to the preamble of thefirst claim, and to a method for the production of a shaped bodycorresponding to the sixteenth claim.

Coatings serve to improve mechanical, electrical, chemical, optical orother material properties on the surface of a component, in order toachieve application advantages, or to prevent or retard negative effectson the component in its use.

The application of dense coatings in the form of glazes on ceramicsubstrates has long been known. The substrate materials are mostlycoarse or refractory ceramics with a correspondingly low level ofmechanical properties and structure. The dense coating is intended tosubstantially mask these disadvantages. For example, chemical stabilityis substantially improved by the glazes.

In the case of coatings on high-performance ceramics, however, theattempt is made to combine the advantageous properties of coatingsubstances with those mechanical and thermal properties of the substratematerial which suffice for the extreme stresses.

For example, coatings of various chemical elements and compounds areused and proven in technology which are applied to the substrate by CVD,PVD, plasma or other such techniques and also combinations of same. Whatis a disadvantage in these methods of application is that they workthrough the gaseous phase which causes the number of materials that canbe used for the coating to be greatly limited. The coating thicknesseswhich can be achieved range from a few μm to about 25 μm and, due to thecoating process, are very costly. With the methods referred to it ispossible only to modify the surface properties. It is not possible,however, to substantially affect the structure of the surfaces.Moreover, the adhesiveness of the coatings depends on the process used.In coating by the plasma process the adhesion of the coating isaccomplished only by adhesive forces, so that the duration of the bondis accordingly limited.

Other thermal and chemical coating methods have the disadvantage thatthe coating process affects the structure of the substrate material andits material properties can even be impaired. On account of the two-stepmethod for the manufacture of a component as substrate followed bycoating, tensions can be produce between the coating and the materialwhich impair the adhesivity of the coating to the substrate,

Sintering ceramic bodies of different porosity to one another is stateof the art, but on account of problems at the boundary surface f thebodies and the internal tensions that occur no complex components can bemade.

Ceramic bodies which consist entirely of an open-pored material arestate of the art. But their mechanical strength is greatly reduced.

By the above-mentioned method, therefore, the production of a coating ofdefined thickness and pore structure on a closely sintered substrate ofan inorganic material is not possible.

The invention is therefore addressed to the problem of preventing theknown disadvantages in the production of a porous coating on a sinteredbody of an inorganic material,

The solution of the problem is accomplished by means of a shaped body,as claimed in the first claim, and a method for the production of abody, especially one according to claims 1 to 15, as it is claimed inclaim 16. Advantageous embodiments of the invention are claimed in thesub-claims.

The invention avoids the disadvantages of the state of the art in theproduction of a body with a porous coating on its surface by the factthat first a base body, the substrate, is shaped as a greenbody from aninorganic material, and a suspension of the same inorganic material ofwhich the substrate consists, or of some other material, is applied tothe substrate in its state as a greenbody. This suspension contains inaddition to the inorganic material a pore-forming substance. Only afterthe application of the coating is a heat treatment of the substance andcoating together performed by drying and sintering to produce amonolithic shaped body. The method for the production of the substrateis not different from the methods known in the state of the art.

The base body can be either free of pores, densely sintered, or also cancontain pores. In the last case it also contains in its state as agreenbody, a percentage of a pore forming substance. Of course, thecontent of this substance is then such that the amount of pores per unitvolume is always greater in the coating than in the substrate.

Suitable as inorganic materials for the base body, the substrate, areespecially ceramic materials such as the known oxide ceramics, alsosilicates, phosphates, apatite and related materials, as well asnitrides, carbides and suicides. It is also possible to make bodies bythe method of the invention with a porous surface coating from metalsproduced by powder metallurgy.

The same inorganic materials are suitable for the production of thecoating which are suitable for the production of the base body. It isindeed advantageous if, in selecting an inorganic material for thecoating which is not identical with the inorganic material of the basebody, to assure that the material of the substrate and the material ofthe coating have nearly the same coefficients of expansion and the samegreat thermal stability in the temperature range to be used for thesintering of the body. This will prevent it from happening that, due tothe different expansion of the various inorganic materials and tochanges in the lattice structure or chemical composition, tensionsespecially in the boundary area between the two materials may lead tothe separation or destruction of the coating,

There is an advantageous effect on the thermal behavior of the bodyduring the sintering process when the grain size of the material of thesubstrate and the grain size of the cover material are the same. If theyare different, there is the danger, especially in the boundary areabetween the base body, the substrate and the coating, that tensionsmight occur which also might result in the separation or destruction ofthe coating.

To enable a porous coating to form on the base body—the substrate—theinorganic material to be used for the coating in a suitable grain sizewith a suitable liquid and a suitable pore forming substance are mixedto form a suspension and this suspension is applied in the necessarythickness to the greenbody. The preparation of a suspension ofaninorganic material in a liquid adjusted to this material, allowing forshrinkage during heat treatment, drying and sintering, and from asubstance appropriate to the size, shape an number of the pores, areknown in the state of the art, for example in DE 44 42 810 A1, DE 44 32477 C2 or the publication, “Einfluss von organischen Verbindungen aufkeramische Massen” [influence of organic compounds on ceramic mixtures],W. Mann, Ber. DKG, 373 (1960), pp 11 to 22.

In the last-named publication a series of methods for the formation ofpores is explained. Accordingly, there are the burn-out method, thesolution method, the sublimation method, the evaporation method, theswelling method, the gassing method and the foaming method.

Suitable pore forming substances are especially organic substances, suchas starches, cellulose or waxes, and natural and synthetic polymerswhich evaporate, turn to gas, are consumed or burn and thereby form thepores. The number of pores per unit volume, their size, that is theirdiameter, as well as their shape, can advantageously be determinedthrough the selection of a suitable pore-forming substance. In the caseof solid substances, the amount of particles, their size and their shapeare the decisive factors. The shape of a solid pore forming substancecan be, for example, spherical, globular, laminar or fiber-like.

As a rule the pore forming substances are converted during the heattreatment of the body to a gaseous phase which, when the gas escapesfrom the body, results in open pores, that is, the pores are joinedtogether. As it can be learned from the last-named publication, thereare also processes, such as gassing and foaming methods, in which thepores remain closed. The nature of the pores depends on the anticipateduse of the body. Open pores are advantageous whenever liquids or gasesare to pass through the body and, for example, additional substances areto be deposited in the pores. Bodies with closed pores are suitable, forexample, for sound and thermal insulation as well as electricalinsulation.

Porosity, i.e., the number of pores per unit volume, can be controlledby the amount of the pore forming substance added, or its concentrationin the case of liquid substances, such that the porosity isapproximately between 25% and 90%, preferably between about 25% and 70%.The pore size, pore diameter, depends in the case of solid substances,particularly on the particle size of the pore building substance and canbe adjusted to sizes between about 1 μm and 1000 μm, preferably between20 μm and 500 μm. It is required that the substances used do not undergoany change in volume during the burn-out or outgassing.

In an advantageous embodiment of the invention, when the coating isapplied to the base body or substrate, while it is a greenbody, themoisture content of the suspension is adapted to the preliminarycompression of the material of the substrate. The lower the preliminarycompression of the substrate is and the higher its moisture content is,the more carefully must the moisture content of the suspension beadjusted so that the substrate will retain its shape and stability whenthe coating is applied. Moreover, the moisture content of substrate andsuspension must be coordinated with one another so that, during thesubsequent heat treatments the shrinkage of substrate and coating willbe approximately the same, so that fissures, deformation or break-up ofthe coating will not occur while they are drying.

The coating materials as well as the pore-forming substances aresuspended in water or other appropriate liquid which is known in thealready-named state of the art, so that the suspension has a consistencysuitable for the application process. Moreover, to produce a suspension,dispersants can be added with which a uniform distribution of the solidswithin the suspension is achieved. By adding organic or inorganicadjuvants the viscosity of the suspension can be controlled. By theaddition of highly wetting liquid, the strength of the adhesion to thesubstrate in the green state can be increased.

What has been stated as advantageous process parameters for preparingthe suspension for application to the substrate equally applies to thepreparation of the substrate itself.

The process for applying the coating to the substrate can advantageouslybe adapted to the geometry and the shape of the surface of the substrateas well as the desired thickness of the coating. The coating can beapplied to the entire surface of the substrate or else only to one ormore portions thereof.

For complex surface structures as well as thin coatings of about 0.02 mmto about 2 mm, the immersion method is especially suitable. Theimmersion method also makes it possible to build up a coating in anumber of immersion steps following one on the other up to the desiredtotal thickness. After each immersion which builds up a coating in acertain thickness, this coating is first dried to a degree appropriatefor the formation of the next coating before the next coating is made.

Especially on planar surfaces the suspension can also be brushed, and inthe case of thick coatings it can be applied with a spatula. Sprayingrequires a sprayable suspension. Sprayed-on coatings have a roughsurface which can be advantageous, for example, in the case of implantsor catalysts. The coatings can also easily be applied successively byspraying. By means of the proposed methods coatings can be appliedranging from about 0.02 mm to 10 mm, preferably about 0.1 mm to 2 mm. Bychanging the properties of the features listed below, as well as thepossible combination of these features, i.e., by different inorganicmaterials in substrate and coating, by different amounts of pores perunit of volume in the substrate and in the coating, by the pore size andpore shape, by the thickness of the coating, the arrangement of thecoating on the surface of the substrate as well as the shape of thesurface of the coating itself, a number of applications can be found forbodies according to the invention, of which a number of examples arelisted herewith:

The bodies according to the invention can be used for example asimplants in medical technology. Medical implants, for example socketinserts for hip joints, are made from high-purity aluminum oxide ceramicfor the sake of good tolerability and biocompatibility as well as goodwear characteristics. With a coating which consists also of aluminumoxide, Al₂O₃ in a thickness of a few tenths of a millimeter and withopen pores with a diameter of about 200 μm to 400 μm, the bone tissue isgiven the possibility of growing onto or into the coating and a directanchoring of the socket in the bone is possible. Instead of coating thesocket as the base body with a porous aluminum oxide coating, it canalso be coated with a coating of hydroxyl apatite or other calciumphosphate compounds in the same thickness and with the same porestructure. Hydroxyl apatite facilitates the growth of the bone tissueinto the pores of the coating of the implant. Hydroxyl apatite can alsoadditionally applied in a thin layer to the porous aluminum oxidecoating.

The following examples indicate possible industrial applications. Onto asilicon nitride substrate, Si₃N₄, an additional coating of siliconnitride is applied so that then a very adherent, active coating withprecursors can follow.

In process technology and in chemistry, porous coatings of siliconcarbide, SiC, on substrates which are also made of silicon carbide,promote the evaporation of liquids due to the enlarged surfaces.

The shaped bodies according to the invention are also suitable ascatalyst supports, The porous coating then serves on the highlyrefractory ceramic substances as support for the catalyst material. Suchcatalysts find application for example in motor vehicles and in thechemical industry. Also, the bodies according to the invention serve forlining containers, pipelines and troughs in metallurgy and in thechemical industry. For example, in the case of surfaces of foundry toolsthat come in contact with molten metals, for example, to protect themagainst corrosion, a porous coating of cordierite on nonporouscordierite or a porous coating of aluminum titanate on nonporousammonium titanate is proposed. Thus the surface tension is increasedagainst the molten metals and the wetting action is reduced.

The invention is explained with the aid of the following embodiments.

FIG. 1 shows a flat body with a porous coating,

FIG. 2 an enlarged microsection of the porous coating and the adjacentbase body,

FIG. 3 an insert cup of a hip joint endoprosthesis with a coatingpromoting the ingrowth of the bone tissue, and

FIG. 4 an enlarged microsection of the porous coating and the contiguousmaterial of the insert cup.

Now the production of a shaped body of silicon nitride, Si₃N₄, accordingto the invention will be described, as it is represented in FIG. 1 andidentified by 1. By the process steps known in the state of the art,silicon nitride is prepared by dispersion in water with the addition ofsoluble binders, by grinding and spray drying to form a pressable dough.The granular product obtained by spray drying is pressed at an axialpressure of 2000 bar to form a square plate 1 with an edge length of 17mm and a height of 7 mm. The embodiment is represented on an enlargedscale in FIG. 1. The density of the greenbody 2 is 1.9 g/cm,corresponding to 60% of the theoretical density of Si₃N₄.

A portion of the aqueous Si₃N₄ dispersion is separated prior to thespray drying. The solid content is about 60 wt.-% (weight percent). 15wt.-% if a starch powder of a grain size between 20 μm and 50 μm isadded to the dispersion. The thick dispersion thus prepared is brushedas coating 3 onto the pressed Si₃N₄ plate, the substrate 2. The watercontent of the applied dispersion is absorbed by the greenbody 2 and theapplied coating 3 solidifies. By repeated brushing the thickness 4 ofthe coating 3 can be established as desired, for example up to 2 mm. Themoisture content of the substrate 2 as the greenbody and of coating 3 isadapted one to the other such that tensions and cracking are avoided inthe drying and in the firing that follows.

The substrates 2, the plate 1, provided with the coating 3, are driedlike conventional greenbodies of silicon nitride, and sintered at theusual sintering temperature of up to 1800° C. The coating 3 sintersmonolithically with the substrate 2. The burnt-out organic matter leavesopen pores 5 behind.

FIG. 2 shows a section through the coating 3 on the plate 1 and the areaof substrate 2 beneath it. The photograph shows a 200-times enlargementby a light microscope. The thickness of the porous coating 3 on theright amounts to about 0.3 mm, an approximately uniform distribution ofcoherent, spherical pores 5 of approximately equal size are clearly tobe seen in the coating 3, and they have a diameter 6 of about 20 μm to30 μm. The amount of pores per unit volume—the porosity—is about 35%.

The marginal layer 7 of the substrate 2 likewise has pores 8 which aresomewhat larger and irregularly arranged than the pores of the porouscoating 3. This effect, which is generally called “sinter skin” inconnection with ceramic materials, has its cause in reactions betweenthe surface and the sintering atmosphere. The marginal layer 7 in thepresent embodiment forms, for example, when silicon nitride is sinteredin the presence of substances which as they decompose give off gasescontaining carbon and oxygen which react with the nitrogen and thesilicon and likewise form gaseous phases, such as SiO and N₂. This hasbeen the case in the sintering of the present embodiment, because thestarch powder has decomposed. The gases which thus formed have reactedwith the material of the marginal layer 7 to form pores. The porositydecreases inward from the surface of the substrate 2. The sinter skincan reach a thickness up to 3/10 mm.

While the so-called sinter skin is removed as a rule by grinding,because its porosity interferes with the otherwise intended purpose ofsintered ceramics, it can even be regarded as desirable in the presentcase, because the pores are thereby opened all the way into the basebody. In the case of the infiltration of these pores, for example, theresult is the possibility of anchoring the porous coating firmly intothe base body, the substrate 2, by means of the infiltrated materials.

In FIGS. 3 and 4 an embodiment from medical technology is represented.FIG. 3 shows a socket insert 10 of a hip joint endoprosthesis made ofaluminum oxide, Al₂O₃, The socket insert 10 consists of the base body 11with the slip surface 12 and the outer surface 13 on which a porouscoating 14, also of aluminum oxide, has been applied. This porouscoating 14 is intended to promote the ongrowth and ingrowth of the bonetissue. The coating 14 has uniformly distributed open pores 15.

The coating 14 is developed out of the material intended for theproduction of the socket insert. 15 wt.-% of a polyethylene wax with agrain size between 100 μm and 500 μm is added to this dispersion. Theviscous dispersion thus prepared is spread onto the outer surface 13 ofthe base body 11, the procedure being as described in the previousembodiment.

FIG. 4 shows a light microscope photograph in a fifty-fold enlargementof a micrograph of the structure of the porous coating 14 and theadjoining base body 11 after sintering. Clearly seen is the base body 11appearing to be pore-free, and its outer surface 13 as a boundarybetween base body 11 and porous coating 14. The specimen taken from asocket insert is embedded in a synthetic resin 16 appropriate for theproduction of photomicrographs. The embedding material 16 appears darkin the micrograph. It has filled the pores 15 and for this reason theycan hardly be seen, especially in the transition to the surface 17 ofthe coating 14. The coating 14 has a thickness 19 of around 1.5 mm and aporosity of about 50%. It consists of the same material as that of thebase body 11, i.e., Al₂O₃.

The round pores 15 of up to 400 μm diameter form a largely cohesivestructure As it can be seen, the result is a very greatly fissuredsurface which advantageously promotes the ongrowth and ingrowth of bonetissue.

1. A shaped body comprising a base body comprising a sintered shapedgreenbody comprising a ceramic material and a porous coating comprisinga ceramic material and a pore forming substance on at least a portion ofthe surface of the base body, wherein the base body and porous coatinghaving a different number of pores per unit volume and the shaped bodyis monolithic; wherein after the shaped body is sintered the coating ismonolithically sintered together with the substrate and wherein thesubstrate has a content of less than 1% of pores per unit volume, andwherein the substrate and the coating consist of different ceramicmaterials.
 2. The shaped body according to claim 1, wherein thesubstrate is completely coated with said porous coating.
 3. A shapedbody comprising a base body comprising a sintered shaped greenbodyceramic material and a porous coating comprising a ceramic material anda pore forming substance on at least a portion of the surface of thebase body, wherein the base body end porous coating having a differentnumber of pores per unit volume and the shaped body is monolithic;wherein after the shaped body is sintered the coating is monolithicallysintered together with the substrate and wherein the substrate has acontent of less than 1% of pores per unit volume, wherein the substrateis completely coated with said porous coating, and the grain size of theceramic material of the substrate and the grain size of the ceramicmaterial of the coating are different and wherein the substrate andcoating comprise different ceramic materials.
 4. The shaped bodyaccording to claim 1, wherein the ceramic material of the substrate andthe ceramic material of the coating have coefficients of expansion ofvirtually equal magnitude and a virtually equal thermal stability in thetemperature range which is necessary for the sintering of the greenbody.5. A shaped body comprising a base body comprising a sintered shapedgreenbody ceramic material and a porous coating comprising a ceramicmaterial and a pore forming substance on at least a portion of thesurface of the base body, wherein the base body and porous coatingcomprise different ceramic materials and have a different number ofpores per unit volume and the shaped body is monolithic; wherein afterthe shaped body is sintered the coating is monolithically sinteredtogether with the substrate and wherein the substrate has a content ofless than 1% of pores per unit volume, wherein the grain size of theceramic material of the substrate and the grain size of the ceramicmaterial of the coating are the same.
 6. The shaped body according toclaim 1, wherein the thickness of the coating on the substrate isapproximately between 0.02 mm and
 10. 7. The shaped body according toclaim 1, wherein the proportion of pores per unit volume in the coatingis approximately between 25% and 90%.
 8. The shaped body according toclaim 1, wherein the diameter of the pores in the coating isapproximately between 1 μm and 1000 μm.
 9. The shaped body according toclaim 1, wherein the shaped body is a medical implant.
 10. The shapedbody according to claim 1, wherein the shaped body is a component of afilter.
 11. The shaped body according to claim 1, wherein the shapedbody is component of a catalyst.
 12. The shaped body according to claim1, wherein the shaped body is component of a foundry tool.
 13. Theshaped body according to claim 1, wherein the shaped body is componentof a cutting tool.
 14. The shaped body according to claim 1, wherein theshaped body serves as a lining of containers, pipelines and troughs inmetallurgy and in the chemical industry.
 15. A method for themanufacture of a shaped body comprising shaping a base body comprisingat least one inorganic material to form a greenbody; applying a coatingcomprising a suspension of a predetermined proportion of a pore formingsubstance and an inorganic material to at least a portion of a surfaceof the base body and subjecting the coated greenbody to a heat treatmentto produce the monolithic sintered shaped body.
 16. A method accordingto claim 15, wherein a pore-forming substance is admixed only to thematerial of the coating to be applied.
 17. A method according to claim15, wherein a coating comprising a different material is applied to thesubstrate than the one of which the substrate consists.
 18. A methodaccording to claim 16, wherein the greenbody is dried or sintered priorto application of the coating.
 19. A method according to claim 16,wherein the moisture content of the suspension is adapted to thepreliminary compression of the material of the substrate that is stillin the green state.
 20. A method according to claim 19, wherein theviscosity, the wetting and drying behavior and the adhesive strength ofthe suspension is adapted to the state of the material of the substratethat is still in the green state.
 21. A method according to claim 15,wherein the application of the material of the coating is performed bydipping.
 22. A method according to claim 15, wherein the application ofthe material of the coating is performed by brushing or troweling.
 23. Amethod according to claim 15 wherein the application of the material ofthe coating is performed by spraying.
 24. A method according to claim21, wherein the coating is applied in several layers.
 25. A methodaccording to claim 15, wherein the coating is applied in a thickness atwhich the shrinkage caused by the heat treatments is allowed for.
 26. Amethod according to claim 15, wherein the material of the coating isapplied in a thickness of about 0.02 mm to about
 10. 27. A methodaccording to claim 15, wherein the substance forming the pores isadmixed to the material of the coating in such an amount orconcentration that when the shaped body is sintered the specifiedproportion of pores per unit of volume is reached, which isapproximately between 25% and 90%.
 28. A method according to claim 15,wherein the particle size of a solid substance forming the pores isadapted to the desired number of pores to be produced, which isapproximately between 1 μm and 1000 μm.
 29. A shaped body according toclaim 2, wherein the substrate has a content of less than 1% of poresper unit volume.
 30. The shaped body according to claim 11, wherein acoating of another material is applied to the substrate than the one ofwhich the substrate consists.
 31. The shaped body prepared by the methodof claim
 15. 32. The shaped body according to claim 6, wherein thethickness of the coating on the substrate is approximately between 0.1mm and 2 mm.
 33. The shaped body according to claim 7, wherein theproportion of pores per unit volume in the coating is approximatelybetween 25% and 70%.
 34. A shaped body comprising a base body comprisinga sintered shaped greenbody ceramic material and a porous coatingcomprising a ceramic material and a pore forming substance on at least aportion of the surface of the base body, wherein the base body andporous coating comprise different ceramic materials and have a differentnumber of pores per unit volume and the shaped body is monolithic;wherein after the shaped body is sintered the coating is monolithicallysintered together with the substrate and wherein the substrate has acontent of less than 1% of pores per unit volume, wherein the diameterof the pores in the coating is approximately between 20 μm and 500 μm.35. A method according to claim 26, wherein the material of the coatingis applied in a thickness of about between 0.1 mm and 2 mm.
 36. A methodaccording to claim 27, wherein the substance forming the pores isadmixed to the material of the coating in such an amount orconcentration that when the shaped body is sintered the specifiedproportion of pores per unit of volume is reached, which, isapproximately between 25% and 70%.
 37. A method according to claim 28,wherein the particle size of the solid substance forming the pores isadapted to the desired number of pores to be produced, which isapproximately between 20 μm and 500 μm.
 38. The shaped body of claim 31,wherein said inorganic material is a ceramic.
 39. The shaped body ofclaim 31, wherein the base body is monolithic.
 40. The shaped body ofclaim 38, wherein the base body is monolithic.
 41. The shaped bodyaccording to claim 6, wherein the proportion of pores per unit volume inthe coating is approximately between 25% and 90%.
 42. A shaped bodycomprising a single base body comprising a sintered shaped greenbodyceramic material and a porous coating comprising a ceramic material anda pore forming substance on at least a portion of the surface of thebase body, wherein the base body and porous coating having a differentnumber of pores per unit volume and the shaped body is monolithic. 43.The shaped body of claim 1, wherein base body has a content of less than1% of pores per unit volume.
 44. The method of claim 15, wherein saidheat treatment is sintering.
 45. A shaped body comprising: a sinteredshaped greenbody ceramic; a porous coating comprising a ceramic on atleast a portion of a surface of said base body; and a porous marginallayer between said base body and said porous coating.
 46. A shapedgreenbody having at least one surface coated with a suspensioncomprising a ceramic and a pore former.
 47. A method comprising applyinga suspension comprising a ceramic and a pore former to at least aportion of a surface of a shaped greenbody.
 48. A monolithic shaped bodycomprising a base body comprising a sintered shaped greenbody comprisinga ceramic material and a porous coating on at least a portion of thesurface of the base body, wherein the base body and porous coating havea different number of pores per unit volume; and wherein the porouscoating is formed by applying a suspension comprising a ceramic and apore forming agent to a green body ceramic and monolithically sinteringthe coated green body to form the monolithic shaped body and wherein thesubstrate and the coating comprise different ceramic materials.
 49. Theshaped body according to claim 48, wherein the substrate is completelycoated with said porous coating.
 50. The shaped body according to claim49, wherein the grain size of the ceramic material of the substrate andthe grain size of the ceramic material of the coating are the same. 51.The shaped body according to claim 48, wherein the ceramic material ofthe substrate and the ceramic material of the coating have coefficientsof expansion of virtually equal magnitude and a virtually equal thermalstability in the temperature range which is necessary for the sinteringof the greenbody.
 52. The shaped body according to claim 48, wherein thegrain size of the ceramic material of the substrate and the grain sizeof the ceramic material of the coating are the same.
 53. The shaped bodyaccording to claim 48, wherein the thickness of the coating on thesubstrate is approximately between 0.02 mm and 10 mm.
 54. The shapedbody according to claim 48, wherein the proportion of pores per unitvolume in the coating is approximately between 25% and 90%.
 55. Theshaped body according to claim 48, wherein the diameter of the pores isapproximately between 1 μm and 1000 μm.
 56. The shaped body according toclaim 48, wherein the shaped body is a medical implant.
 57. The shapedbody according to claim 48, wherein the shaped body is a component of afilter.
 58. The shaped body according to claim 48, wherein the shapedbody is component of a catalyst.
 59. The shaped body according to claim48, wherein the shaped body is component of a foundry tool.
 60. Theshaped body according to claim 48, wherein the shaped body is componentof a cutting tool.
 61. The shaped body according to claim 48, whereinthe shaped body serves as a lining of containers, pipelines and troughsin metallurgy and in the chemical industry.
 62. A method according toclaim 16, wherein the greenbody is dried prior to application of thecoating.
 63. A shaped body comprising a base body comprising a sinteredshaped greenbody comprising a ceramic material and a porous coatingcomprising a ceramic material and a pore forming substance on at least aportion of the surface of the base body, wherein the base body andporous coating having a different number of pores per unit volume andcomprise different ceramics; wherein the shaped body is monolithic;wherein after the shaped body is sintered the coating is monolithicallysintered together with the substrate and wherein the substrate has acontent of less than 1% of pores per unit volume.
 64. A sintered shapedbody prepared by the process of: forming a green body comprising a firstceramic material; applying a coating comprising a second ceramicmaterial and a pore forming substance to the green body to form anunsintered shaped body; sintering the unsintered shaped body to form thesintered shaped body, wherein the coating is porous and wherein thegreen body, after sintering, forms a substrate having less that 1% poresper unit volume and wherein the first and second ceramic material aredifferent.
 65. The sintered body of claim 64, wherein the first andsecond ceramic material have coefficients of expansion of virtuallyequal magnitude.
 66. The sintered body of claim 65, wherein the firstand second ceramics have virtually equal thermal stability in thetemperature range at which the sintering occurs.
 67. A shaped bodycomprising a sintered greenbody comprising a first ceramic and porouscoating comprising a second ceramic that have been sintered together,wherein the sintered greenbody has a content of less than 1% of poresper unit volume and the coating and the sintered greenbody aremonolithic wherein the first and second ceramics are different.
 68. Themethod of claim 47, further comprising sintering the greenbody to whichthe suspension has been applied to form a sintered shaped bodycomprising a porous coating formed from the sintered suspension and asintered greenbody.
 69. The method of claim 68, wherein the coating andsintered greenbody are monolithic.
 70. The method of claim 69, whereinthe porous coating has a porosity, but said porosity is less than 1%pores per unit volume.
 71. The shaped body according to claim 63,wherein the coefficient of expansion of the shaped body and thecoefficient of expansion of the coating are different.